The present invention relates in general to laser processing of workpieces such as semiconductor devices and more particularly concerns processing of DRAMS, memories, and programmable devices by cutting fuses or links.
Laser systems have been used for many years in the fabrication of DRAMS and programmable devices. In DRAM production, for example, redundant memory is programmed by using a focused laser beam to cut fuses or links in the memory in order to replace defective memory cells. The programming is accomplished by disconnecting the fuses or links using a laser pulse generated by a diode pumped Q-switched YAG (or YLF) laser.
Semiconductor devices have link geometries typically about 1 microns wide by 5 microns long, but the trend is toward finer geometry to support increasingly high density DRAM devices, for example a link dimension of 0.6 microns×5 microns. These links may be located in groups of horizontally aligned links and vertically aligned links. A laser having 3-5 micron laser spot size has often been used to disconnect such a link using a single laser pulse. By appropriately selecting the laser energy, the spot size, the laser pulse width, and the wavelength of the laser beam, it is possible to optimize laser parameters in order to achieve the cleanest and most reliable link disconnect.
The quality of a link disconnect may be evaluated by visually inspecting the blasted link. One measure of practicality in fuse or link disconnect is the energy cutting range or “energy window,” which is the range of energies per pulse over which clean and reliable link cutting can be achieved. The laser energy that is used to process a semiconductor device can be set at the center of the predicted energy window, which may differ somewhat from the actual energy window due to process variations such as the thickness of the link material, the thickness of oxide material located on top of the link, laser instability, errors in the positioning of the laser beam, and focusing errors.
Polysilicon has been widely used for the link material for the past years due to its superior cut quality. Material properties, such as the deep absorption in the 1 μm wavelength range, provides relatively uniform temperature distribution. This relatively uniform temperature distribution promotes clean removal of link material by laser irradiation. However, the high resistance and complex processing of polysilicon limits its use in deep sub-micron application (See J. B. Bernstein, Y. Hua, W. Zhang, “Laser Energy Limitations for Buried Metal Cuts”, IEEE Electron Device Letters, Vol. 19, No. 1, pp. 4-6, 1998).
Aluminum fuses became a new candidate to replace polysilicon and have been studied recently for their manufacturability and reliability. Various failure mechanisms, including lower corner cracking and material remaining at the bottom of the cut site, have been investigated and set the high and low bounds, respectively, of the laser energy window (see J. B. Bernstein, J. Lee, G. Yang, T. Dahmas, “Analysis of Laser Metal-Cut Energy Process Window,” IEEE Semiconductor Manufacturing, Vol. 13, No. 2, pp. 228-234, 2000). Furthermore, collateral damage to the adjacent fuse structures or substrate due to excessive energy and laser spot positioning error is also another failure mode at high laser energy.
More recently, for high-performance logic devices and high-speed SRAM, copper has been investigated as link material due to its enormous benefits when compared to aluminum, such as its low resistance, power dissipation, manufacturing cost, and superior resistance to electromigration. However, there have been found some difficulties in the laser processing of copper fuses because of the different material properties and fabrication of copper metallization, such as lower coefficient of thermal expansion and higher melting point, as well as its thick structure.
Many diode-pumped solid-state lasers used in laser processing systems are linearly polarized. Certain laser processing systems use circularly polarized laser beams rather than linear polarized laser beams.